1. Technical Field
This invention relates generally to position sensing systems and, more particularly, to a contact sensor position measurement control system and method which provides constant contact force control.
2. Discussion
Position measurement sensors are commonly employed to perform measurement operations to measure a physical variable of position and/or displacement (i.e., change of position) of a desired surface. Generally speaking, some of the more precise conventional position measurement sensors are also known as measurement transducers and generally include a moveable mechanical probe that extends and retracts relative to the surface being measured. In response to such movement, the position sensors typically generate an electrical signal which represents the movement of the probe in relation to the surface and thereby defines the measured position or displacement thereof.
A wide number of position measurement sensors currently exist which include both contact and non-contact probe arrangements. .Non-contact position sensors include the use of interferometry in which an optical beam is transmitted from a light source (probe) through a lens which divides and transmits half the beam to the surface to be measured and the remaining half of the beam to a reference optical surface. Each beam is reflected back to an optical measurement device such as a radiometer with necessary electronics. The measurement device operates to detect interference fringes produced by the two reflected beams which are then counted and analyzed in order to determine the separation distance therebetween. The separation distance thereby enables the user to determine the position thereof and the displacement between the probe and the surface being measured.
In the past, interferometry has been known to provide highly accurate position measurement results. However, the more recently developed systems employing interferometry are generally overly complex and expensive. Less expensive optical systems do exist which use a fiber optic probe for transmitting an optical beam off of the surface to be measured and measuring the reflected light beam intensity as a function of the position of the probe. However, the commercially available optical sensors are generally only capable of operating with precision and repeatability over a very limited linear range. In addition, optical sensors usually have relatively low reliability and difficult calibration procedures. Furthermore, optical sensors are easily susceptible to interference and are especially sensitive to contamination problems.
Other types of conventional non-contact position sensors include capacitance transducers and eddy current based sensors. The capacitance transducer technique generally requires a conductive surface which forms a capacitive coupling with a separate but closely spaced plate. The eddy current sensors require magnetically induced circuits on the probe and surface being measured. However, these non-contact sensors frequently suffer from low reliability and it is therefore difficult to ensure highly accurate measurements when using such devices. In addition, these types of non-contact sensors are easily susceptible to damage or fault caused by corrosion among other causes.
A number of contact position sensors are currently available for measuring position and/or displacement in response to the distance a probe is moved in which the probe directly contacts the measured surface. Contact position sensors usually include a mechanically actuated spring-loaded probe for forcibly contacting the measured surface. In the past, contact sensors have included a linear potentiometer to directly measure the distance the probe is moved. However, linear potentiometers are basically unsophisticated devices which generally provide poor to moderate accuracy at best.
More recently, a more enhanced contact sensor has been developed which is known as a linear variable differential transformer (LVDT) sensor such as the type manufactured by Schaevitz. LVDT sensors include a transformer which has a movable core disposed in a region between first and second sets of coils. The first set of coils is excited with an alternating current (AC) signal, while the second set of coils receives an induced voltage in response to the AC signal which is based on the position of the movable core. The movable core is axially moved in response to the position of a spring-loaded probe which forcibly contacts the surface under measurement. The LVDT sensor generally provides accurate position measurement, however, the amount of force which results between the probe and the surface under measurement varies according to the position of the probe and compression of the spring among other factors.
While the above-described LVDT sensors have traditionally provided adequate measurement capabilities for a number of applications, there exists a need for a highly precise laboratory measurement device which can exhibit a small controlled amount of force. In particular, there currently exists a need for a precision contact position sensor that would enable a user to measure the position of precision polished optics in which the contact force exerted upon the lens is extremely small. The extremely small contact force is necessary to prevent the probe from exceeding sensitive contact pressures which may damage optics such as the type currently found on large X-Ray telescopes. Thus, one would be able to perform position measurement operations on the surface of the precision polished optics for such purposes as optic engagement, tilt orientation, decenter, metrology engagement and optic disengagement operations as well as emergency proximity sense control without damage to the optic, especially during translation and rotation of the optics.
It is therefore desirable to provide for a contact position sensor which is able to maintain a small controlled amount of force exerted between the contact probe and the surface to be measured. In addition, it is further desirable to provide for a method and system for controlling a contact position sensor so as to achieve the desired amount of force. In particular, it is desirable to provide for such a sensor which enables one to perform measurement operations on optics where the force remains a selected constant and has a very small magnitude. It is further desirable to provide for such a sensor which operates over a wide range and exhibits substantially linear operation.